Los Angeles County firefighters knocked down a fire in a Chevrolet Blazer on the shoulder of northbound Interstate 5 near the Magic Mountain Parkway on-ramp at about 3:45 p.m. Friday.

The three occupants of the car, among them a 3-year-old girl, escaped the blaze without injuries, authorities said on the scene.

An overheated transmission is suspected to be the cause, said Geno Harger, who, along with girlfriend Jenny Gaggero and 3-year-old Emy, watched from a distance as firefighters arrived to handle the fully involved vehicle.

A passerby stopped to try to assist with a fire extinguisher before authorities arrived. He remained with the trio to help continue their trip home to Bakersfield.

The northbound lanes of the freeway remained open as firefighters and California Highway Patrol officers investigated the blaze.